PURPOSE: To reliably prevent the occurrence of a trouble with high responsiveness through simple structure, since when, during low rainfall and very low rainfall, drainage is effected through rated operation of a prime mover, a water level in a suction well is rapidly lowered and a need to immediately stop operation occurs and in the worst case, air is sucked in a pump and high vibration and noise are generated and various adverse influences are exercised on a drainage pump and high capacity drainage pump equipment.
CONSTITUTION: The output rotary shaft 3A of a reduction gear 3 is coupled to the main shaft 2A of a drainage pump 2 and first and second variable speed fluid couplings 9 and 10 are located in series between the input rotary shaft 3B of the reduction gear 3 and the output rotary shaft 6A of a prime mover 6 consisting of a diesel engine.